The functional track directory (FTD) is the collective name for two tables that together map each functional track to an area in the RVA′s physical storage:
−Functional track table
The functional track table (FTT) contains the
−Track number table
The track number table (TNT) contains the
Although the FTD consists of two tables, we discuss it as one entity (see Figure 1). In fact, each functional track has an entry in the FDT. If a functional track contains data, its associated FTD entry has a pointer to the FBA block where the data starts. The data of a functional track may fit on one or more FBA blocks.
If the data of a functional track is updated, the RVA puts the new data in a new place in the log structured file, and the FTD entry points to the new data location. There is no update in place.
Figure 1. The RAMAC Virtual Array Tables
1.1.3 Data Compression and Compaction
All data in the RVA is compressed; compression occurs when data enters an RVA. In addition, the gaps between the records, as they appear on the CKD devices, are removed before the data is written to disk. Similarly, the unused space at the end of a track is removed. This is called compaction. The compression and compaction ratio depends on the nature of the data. Experience shows that RVA customers can achieve an overall compression and compaction ratio of 3.6:1.
Data compression speeds up the transfer from and to the arrays. It increases the effectiveness of cache memory because when compressed more data can fit in cache.
2 RAMAC Virtual Array,
